This invention generally relates to braze materials and brazing processes, such as for use in the manufacturing, joining, coating, repair, and build-up of superalloy components. More particularly, this invention relates to a brazing process that utilizes microwave radiation and the generation of a plasma to promote localized heating and melting of a braze alloy.
Nickel, cobalt, and iron-base superalloys are widely used to form high temperature components of gas turbine engines. While some high-temperature superalloy components can be formed as a single casting, others are preferably or necessarily fabricated by other processes. As an example, brazing can be used to fabricate certain gas turbine components, such as high pressure turbine nozzle assemblies. Brazing is also used to repair cracks and other surface flaws and damage, build up surfaces to restore desired dimensions, and form protective coatings on gas turbine engine components. Brazing techniques of these types encompass heating a braze alloy, typically in the form of a braze alloy powder, a paste or tape containing a braze alloy powder, or a sintered preform of a braze alloy powder, to a temperature above the melting point of the braze alloy but sufficiently below the melting point of the material being brazed to avoid damaging and/or reducing desired properties of the material. (As used herein, “melting point” is meant to encompass the incipient melting point for alloys that do not have a true melting point but instead have a melting range.) For example, brazing temperatures are often limited to avoid grain growth, incipient melting, recrystallization, and/or unfavorable phase formation in the alloy or alloys being brazed.
In situations where a brazement must have a composition and properties similar to the substrate being brazed, the braze alloy will typically have a composition essentially or nearly the same as the substrate, but modified to contain one or more melting point suppressants, such as boron and/or silicon, which form low melting eutectics with the substrate material. In the past, braze alloy powders have been prepared by combining their alloying constituents through such processes as atomization and mechanical alloying to yield a powder whose particles have a uniform composition. For example, EPO456481 reports a process in which a titanium-based braze alloy powder is formed by mechanically alloying powders of each elemental constituent of the braze alloy, including powders of nickel and/or copper as the melting point depressant(s), to create a presumably uniform distribution of the elements in the braze alloy powder.
A difficulty encountered when brazing certain alloys is the tendency for some melting point depressants to form embrittling phases, such as chromium borides that form when brazing chromium-containing superalloys. As a result, brazing is not an appropriate manufacturing or repair process for some applications, particularly many components in the hot gas path of a gas turbine engine. In any case, the amounts of melting point depressants contained in a braze alloy are intentionally limited and sometimes partitioned to minimize their detrimental effects. An example of the latter is the use of a braze alloy system comprising two braze powders, one containing one or more melting point depressants and the other nominally having the same composition as the component being brazed. The higher-melting powder acts as a sink for the excess melting point depressants in the lower-melting powder during and after the brazement is formed. However, segregation of the two powders can occur during the brazing process, with the lower-melting powder taking most of the working volume of the brazement and displacing the higher-melting powder. If this occurs, an excess of melting point depressants will be present in the brazement, which in turn affects the mechanical properties of the brazement.
Microwave brazing is currently been investigated as a potential candidate for eliminating issues associated with conventional brazing techniques. For example, microwave heating has the potential for localizing heat in the braze alloy and selected areas of a component, which reduces the risk of degrading desired properties of the component. Two approaches have generally been proposed for microwave brazing. A first entails the use of a susceptor (e.g., SiC enclosure) that is heated when exposed to microwave energy and, in turn, transfers the heat to the component by radiation. Drawbacks to this approach are lack of local heating of the braze alloy only, as an entire region of the component is inevitably heated, and significant heat loss from radiation in directions away from the intended brazement. A second approach entails direct microwave heating of a braze alloy powder, which is more susceptible to absorbing microwave energy than bulk metals that generally reflect microwaves. Because typical braze alloy compositions do not couple sufficiently with microwave energy to become fully melted during a microwave treatment, braze powders have been proposed that contain particles that are sufficiently small to be highly susceptible to microwave radiation. This approach is disclosed in commonly-assigned U.S. patent application Ser. Nos. 11/564,898, 11/609,473, and 11/469,567. Another approach is to produce a braze powder containing one or more microwave coupling enhancers that are more highly susceptible to microwave radiation than the base alloy composition of the braze powder. For example, commonly-assigned U.S. Pat. No. 7,326,892 to Cretegny et al. discloses the addition to a braze alloy powder of materials capable of behaving as microwave coupling enhancers, such as silicon, germanium, gallium, cobalt, iron, zinc, titanium, carbon (e.g., carbon nano-tubes or fine graphite powder), aluminum, tantalum, niobium, rhenium, hafnium, molybdenum, and silicon carbide (SiC). Powders of the microwave coupling enhancers can be intermixed with a powder of the braze alloy, or the braze alloy can be alloyed to contain one or more microwave coupling enhancers.
Even with the above advancements, there is an ongoing desire to further develop processes for heating braze powders by microwave radiation. In particular, it would be desirable if a braze alloy powder could be fully melted by localized microwave radiation without the presence of microwave coupling enhancers admixed with the powder. In doing so, any impact that such enhancers might have on the mechanical properties of the resulting brazement could be eliminated. In addition, such a capability would allow for broader use of brazing processes and technology, especially in the manufacture and repair of gas turbine engine components.